U.S. patent number 4,740,708 [Application Number 07/000,719] was granted by the patent office on 1988-04-26 for semiconductor wafer surface inspection apparatus and method.
This patent grant is currently assigned to International Business Machines Corporation. Invention is credited to John S. Batchelder.
United States Patent |
4,740,708 |
Batchelder |
April 26, 1988 |
Semiconductor wafer surface inspection apparatus and method
Abstract
A system and procedure for the inspection of the surface of a
semiconductor wafer ascertains that particulate contaminants have
been adequately cleaned from the surface during the manufacture of
integrated electric circuits. The wafer is advanced in a first
direction and is optically scanned in a second direction,
transverse to the first direction, for recording intensities of
light reflected normally from the wafer surface as a function of
location on the scan line. A high intensity reflection is
indicative of a smooth flat surface suitable for inspection of
particles by an integrating hemisphere with plural photodetectors
therein. A weak reflection is indicative of undulations and
patterned regions which are unfavorable for examination of
particles on the wafer surface. A second scan is offset sideways to
compensate for motion of the wafer so as to rescan the same line as
the first scan. The photodetectors in the integrating sphere are
gated on and off during the second scan at the locations of
suitable inspection sites determined from the first scan.
Inventors: |
Batchelder; John S. (Tarrytown,
NY) |
Assignee: |
International Business Machines
Corporation (Armonk, NY)
|
Family
ID: |
21692727 |
Appl.
No.: |
07/000,719 |
Filed: |
January 6, 1987 |
Current U.S.
Class: |
250/559.41;
250/559.16; 356/237.3 |
Current CPC
Class: |
G01N
21/94 (20130101); G01N 2201/0655 (20130101); G01N
2021/8867 (20130101); G01N 21/9501 (20130101) |
Current International
Class: |
G01N
21/88 (20060101); G01N 21/94 (20060101); G01N
21/95 (20060101); G01N 021/88 () |
Field of
Search: |
;250/572,562,563
;356/237,239,430,431 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
IBM Technical Disclosure Bulletin, "Patterned Wafter Scanner", J.
S. Batchelder, vol. 27, No. 10B, Mar., 1985, pp.
6273-6275..
|
Primary Examiner: Westin; Edward P.
Attorney, Agent or Firm: Perman & Green
Claims
What is claimed is:
1. A surface inspection system for determining the presence of
particles on the surface of a wafer, the surface having smooth and
patterned regions, the system comprising:
means for scanning said surface to locate smooth regions
thereof;
means coupled to said scanning means for sensing the presence of
particles resting on said surface; and
means for activating said sensing means, said activating means
being responsive to surface data including locations of smooth
regions outputted by said scanning means for activating said
sensing means in the presence of smooth regions, and for
deactivating said sensing means in the presence of patterned
regions.
2. An inspection system according to claim 1 wherein said scanning
means includes an optical system having an output lens assembly for
illuminating said wafer surface with a beam directed normally to
said surface, said optical system including means for extracting
rays of said beam reflected normally from said surface to sense a
smoothness of said surface.
3. An inspection system according to claim 2 wherein said optical
system includes means for focusing extracted rays of said beam to
obtain an image of said surface, thereby to distinguish a flat
surface from elevations and depression at altitudes different from
a reference altitude of said surface, flat smooth regions of said
surface being at said reference altitude.
4. An inspection system according to claim 2 wherein said scanning
is accomplished as a set of parallel scan lines; and wherein
said scanning means includes a conveyor means for transporting the
wafer past said lens assembly in a direction transverse to said
scan lines; and wherein said scanning means includes means for
offsetting alternate scan lines to compensate for movement of said
wafer to provide pairs of identical scan lines.
5. An inspection system according to claim 4 wherein said
activating means comprises driving means and memory means, said
driving means driving said memory means to store surface data
outputted by said scanning means during a first scan of each pair
of scan lines, said driver means operating said sensing means to
sense particles only during a second scan line of each pair of scan
lines.
6. An inspection system according to claim 5 wherein data stored in
said memory means during a first scan line of said pair of scan
lines is outputted by said memory means during a second scan line
of said pair of scan lines, and wherein said driving means is
activated by said outputted data to gate said sensing means at
specified locations designated by the data outputted from said
memory means.
7. An inspection system according to claim 6 wherein said driver
means includes discriminating means for evaluating data outputted
by said memory means during said second scan line, said driver
means including logic means responsive to signals outputted by said
discriminator means to accomplish said gating of said sensing
means.
8. An inspection system according to claim 7 wherein said sensing
means includes a reflecting hemisphere for integration of radiant
energy in rays of radiation reflected from one of said particles;
and wherein
said optical system includes means for imparting circular
polarization to radiation of said beam for improved detection of
said particles.
9. An inspection system according to claim 8 wherein said sensing
means includes a plurality of detectors of radiant energy directed
inwardly of said hemisphere for reception of rays of radiant energy
reflected within said hemisphere, each of said detectors being
gated by said driver means.
10. An inspection system according to claim 1 wherein said smooth
regions are flat, and wherein said sensing means includes a
reflecting hemisphere for integration of radiant energy in rays of
radiation reflected from one of said particles; and wherein
said sensing means includes a plurality of detectors of radiant
energy directed inwardly of said hemisphere for reception of rays
of radiant energy reflected within said hemisphere; and wherein
said optical system includes means for imparting circular
polarization to radiation of said beam for improved detection of
said particles.
11. A surface inspection system for determining the presence of
particles on the surface of a workpiece, the surface having smooth
and patterned regions, the system comprising:
means for scanning said surface in a sequence of paired line scans,
each pair of line scans including a first scan and a second scan
identical to said first scan;
a surface sensor operatively coupled to said scanning means for
distinguishing between features of the surface;
a particle sensor operatively coupled to said scanning means for
detecting the presence of a particle resting on the surface;
and
activating means operating concurrently with said scanning means
for receiving data of features of the surface from said scanning
means and said surface sensor during said first scan in each pair
of line scans, said activating means activating said particle
sensor during said second scan in each pair of line scans to be
responsive to the presence of a particle located on a flat smooth
region of the surface, said activating means deactivating said
particle sensor in the absence of a flat smooth region of the
workpiece surface.
12. An inspection system according to claim 11 wherein said surface
features include flat regions and patterned regions, said surface
sensor including means for discriminating between flat and
patterned regions.
13. An inspection system according to claim 11 wherein said
scanning means comprises conveyor means for conveying a workpiece
in a direction transverse to the direction of a line scan, and
means for offsetting said second line scan to compensate for
movement of said conveyor means such that said second line scan
coincides with said first line scan.
14. A surface inspection system according to claim 13 wherein said
surface sensor comprises optical beam focusing means including lens
elements and an acousto-optic modulator positioned serially with
lens elements of said focusing means; and wherein
said line offset means comprises means for generating acoustic
signals at a plurality of frequencies for driving said modulator to
deflect a beam of radiation propagating through said optical
focusing means to said surface, there being individual ones of said
acoustic frequencies corresponding to selected offset directions of
said beam.
15. An inspection system according to claim 11 wherein said
activating means comprises a memory for storing location data
received from said scanning means, said location data providing the
locations of positions on said surface scanned within said first
scan, said memory being coupled to said surface sensor for storing
signals of radiant energy reflected from said surface to designate
a type of surface present at each of said locations.
16. An inspection system according to claim 11 wherein said surface
sensor comprises a laser and optical beam focusing means
successively scanning beams of said laser across the surface of
said workpiece, said focusing means directing said beam normally to
said workpiece surface, and wherein said focusing means comprises
beam splitting means for receiving retroreflected rays from said
surface and detecting means operatively coupled to said beam
splitting means for detecting retroreflected rays of radiation,
said detecting means including a lens and a pinhole for producing a
confocal sensing of the surface.
17. An inspection system according to claim 16 wherein said
particle sensor comprises a hemispherical reflector enclosing said
workpiece, there being a plurality of photomultiplier tubes
extending inwardly of said hemispherical reflecting surface for
viewing energy of rays scattered from a particle on the workpiece
surface.
18. An inspection system according to claim 17 wherein said beam
focusing means comprises a scanning mirror in optical alignment
with said beam splitter means, there being a quarter waveplate
producing circular polarization disposed between said scanning
mirror and said beam splitting means for polarizing a beam of
radiation emanating from said laser.
19. An inspection system according to claim 18 wherein said surface
sensor and said particle sensor are optically coupled to said beam
focusing means, said surface sensor and said particle sensor each
comprising a discriminator for comparing amplitude of received
radiation with a threshold, and wherein said particle sensor
further comprises a coincidence detector for detecting coincidence
between optical signals received at each of said photomultiplier
tubes.
20. An inspection system according to claim 18 wherein said
scanning means comprises a shaft angle encoder, and wherein said
activating means comprises an analog shift register and a counter,
said shaft angle encoder being coupled to said scanning mirror and
providing clock pulses for driving said shift register and said
counter, said counter providing a series of counts measuring
locations along a line scan, said shift register being coupled to
said surface sensor for storing data reflected from the workpiece
surface at locations designated by a count of said counter.
21. An inspection system according to claim 20 further comprising
means coupled to said activating means and to said particle sensor
for displaying amounts of energy reflected from a particle upon the
workpiece surface and the locations of such reflections.
22. A method of inspecting a surface of a workpiece to determine
the presence of particles thereon, the method comprising the steps
of:
advancing the workpiece in a first direction;
optically scanning a surface of the workpiece in a second direction
transverse to a said first direction, said optical scanning
employing a beam directed perpendicular to the surface of the
workpiece;
obtaining gross surface data in the form of locations of
undulations and flat portions of the surface of the workpiece;
repeating a scan of the workpiece surface to cover the same region
covered by the first scan of the workpiece surface;
obtaining particle data by light reflected from a particle on the
workpiece surface at locations of flat portions of the surface
obtained from the gross data; and
continuing with the advance of the wafer in the first direction and
the scanning in the second direction to obtain further data of the
surface of the workpiece.
Description
BACKGROUND OF THE INVENTION
This invention relates to a system and procedure for inspection of
a surface of a workpiece and, more particularly, to the inspection
of the surface of a semiconductor wafer for contamination by
particles which may develop in the manufacture of integrated
electric circuits.
Integrated circuits are manufactured on relatively large wafers
which are cut by a saw to separate the individual circuit chips.
Portions of the surface of the wafer may be built up with layers of
material and may also be etched, particularly at the sites of
circuit chips, to impart a pattern to the surface, which pattern is
characterized by undulations in the wafer surface. Other portions
of the wafer surface, particularly regions between the circuit
chips reserved for the kerf of the saw, the saw-cut regions being
the kerf, may be bare of an undulation pattern and have the
characteristics of a specularly smooth surface.
During the process of manufacturing the wafers, it is a common
occurrence for particles of various materials employed in the
manufacturing process to come to rest upon the wafer surface. Such
particles alight on both the smooth and the patterned portions of
the surface of a wafer. Such particles act as a contaminant, and
would interfere with the proper operation of the electric circuits
if allowed to remain on the surface. Accordingly, one step in the
manufacturing process is the cleaning of the wafer surfaces to
remove particulate contamination. Thereafter, it is necessary to
inspect the wafer surfaces to insure that they have been cleaned
adequately of the contamination.
Of particular interest herein is inspection of a wafer surface by
optical inspection equipment. The operation of such equipment is
based on the observation that a reflection of illuminating light
from a wafer surface depends on the direction of illumination
relative to the wafer surface, and also on physical characteristics
of the surface. Specific physical characteristics of the surface
affecting reflection of the illuminating light are smooth regions,
undulating regions, and particles. The smooth regions can produce
high intensity reflections in a specific direction. The undulating
regions can produce both strong reflected signals at specific
angles as well as intense scatter in many directions. A particle
induces a reflection which is relatively weak and is scattered in
many directions.
A problem arises in that equipment which has been configured to
process the weak scattered reflected light from a particle can be
rendered inoperative by exposure to the intense scatter from a
smooth or patterned region. In the case of a smooth surface region,
free of undulations and illuminated by a beam of light normal to
the surface, a path of reflected light can be predicted because it
is known that the reflected light will be also normal to the
surface. However, the problem of dealing with reflected light is
compounded in the case of the undulations of a patterned region
because the surface thereof can reflect light in any one of many
possible directions depending on the local orientation of a part of
the surface receiving an incident beam of illuminating light.
To avoid excessive reflected light in the use of optical inspection
equipment, one form of equipment illuminates the wafer surface
normally, and views reflected light at a glancing angle by light
detectors positioned at or near the surface. Another form of
equipment provides for a viewing of a surface defect along a normal
to the surface in response to both horizontal and vertical
illumination of the subject.
SUMMARY OF THE INVENTION
The foregoing problem is overcome and other advantages are provided
by a procedure and apparatus, in accordance with the invention, for
optically scanning a wafer to determine the locations of surface
regions suitable for the inspection of particulate contaminants
and, by use of the same scanning apparatus, to examine the surface
for particles. Scanning is accomplished in two dimensions by
physically moving the wafer in a longitudinal direction by a
conveyor, and by optically scanning the wafer surface in a
transverse direction perpendicular to movement of the conveyor. A
feature of the invention is the repetition of a transverse scan,
without interruption of the conveyor motion, whereby suitable
inspection sites are selected during the first of a pair of
transverse scans and inspection of the selected sites is
accomplished during the second of the pair of transverse scans. The
repetition of the scan is accomplished in a preferred embodiment of
the invention by use of an acousto-optic modulator which offsets
the path of a transverse scan to compensate for displacement of the
wafer by the conveyor.
Surface information is obtained during the first of the two
transverse scans by illumination of the surface with a light beam
directed by a lens assembly normally to the surface followed by
retroreflection via the lens assembly to a beam splitter and
focusing optics. A reflecting hemisphere is placed between the
surface and the lens assembly to enclose the portion of the wafer
being viewed, the hemisphere having a central slot for admitting
the scanning beam to the surface. Particle information is obtained
during the second of the two transverse scans by plural
photodetectors extending into the sphere for sensing the presence
of light scattered by a particle about the inside of the sphere.
Electronic circuitry is provided for memorizing the locations of
suitable inspection sites, having smooth surface regions, obtained
during the first transverse scan, and for activating the
photodetectors only when these sites appear during the second
transverse scan so as to protect the photodetectors from the
intense light of the patterned regions. It is assumed that the
distribution of the particles is substantially uniform over the
entire wafer surface so that a sampling of particles only in the
smooth surface regions is representative of the adequacy of the
entire wafer cleaning operation.
BRIEF DESCRIPTION OF THE DRAWING
The aforementioned aspects and other features of the invention are
explained in the following description, taken in connection with
the accompanying drawing wherein:
FIG. 1 is a stylized view of an inspection system of the invention,
including wafers carried by a conveyor past an inspection
station;
FIG. 2 is a partial isometric view of an optical system included
within the inspection system of FIG. 1;
FIG. 3 is a schematic diagram of the optical system;
FIG. 4 is a schematic diagram of an electronics unit of the
inspection system;
FIG. 5 shows a wafer with a pair of scan lines indicated
diagrammatically, one scan line being offset to compensate for
translation of the wafer; and
FIG. 6 is a block diagram showing method steps in the procedure by
which the inspection system operates.
DETAILED DESCRIPTION
With reference to FIG. 1, there is shown a surface inspection
system 20 which is constructed in accordance with the invention.
The system 20 comprises an optical scanner 22 and a conveyor 24
which carries semiconductor wafers 26 to an inspection station 28
within the scanner 22. The conveyor 24 moves the wafers 26 in the
direction of an arrow 30 from an input cassette 32 to an output
cassette 34 via the scanner 22, the wafers being carried along a
track 36 which moves continuously during inspections of the wafers
26 by the scanner 22. The conveyor 24 includes a motor 38 which
drives the track 36 at a speed selectable by a person operating the
system 20.
The system 20 provides for the sequential inspection of the wafers
26 in automatic fashion. The wafers 26 are applied, one at a time,
to the scanner 22 which scans the surfaces of the respective wafers
26, in accordance with the invention, to locate particulate
contamination on these surfaces. The results of the surface
inspection of each of the wafers 26 is displayed on an analyzer 40
of optical signals generated within the scanner 22, as will be
described hereinafter.
Each wafer 26 obtained from the input cassette 32 is secured in
position on the moving track 36 by a vacuum chuck 42 (one of which
is shown in FIG. 1) the chuck 42 being carried by the track 36
through the inspection station 28. The scanner 22 is provided with
two light traps, one light trap 44 being located at an input side
of the scanner 22, and the second light trap (not shown) being
located at the output side of the scanner 22. The light traps 44
prevent entry of external light to the interior of the scanner 22,
and also prevent egress of laser light from the scanner 22 so as to
protect personnel from the laser light. Also included within the
system 20 is a console 46 by which an operator can activate the
scanner 22 and the conveyor 24.
With reference also to FIG. 2, there are shown components located
within the interior of the scanner 22. The scanner 22 comprises an
optical integrating hemisphere 48 having two optical detectors 50
and 52, mounted thereon and extending to the interior of the
hemisphere 48 for viewing light scattered from a particle on the
surface of a wafer 26. In a preferred embodiment of the invention,
the detectors 50 and 52 are constructed as photomultiplier tubes
which can be gated on and off by an electrical signal. Thereby, the
detectors 50, 52 can be activated for viewing scattered light from
a particle on the surface of a wafer 26 while being deactivated, so
as to protect the detectors 50, 52 from high intensity light which
may be reflected towards the detectors 50, 52 by undulations in a
patterned region of the wafer surface.
The hemisphere 48 is positioned above the track 36 a sufficient
distance to clear the chuck 42 and the wafer 26 within the
inspection station 28 as shown in FIG. 2. The diameter of the
hemisphere 48 is sufficient to permit the hemisphere 48 to enclose
the portion of the upper surface of the wafer 26 being inspected at
the station 28. The hemisphere 48 is provided with a slot 54
oriented transversely to the direction of movement of the track 36,
the slot 54 having a sufficient length to allow entry of a scanning
laser beam for illuminating the wafer 26. The hemisphere 48 is
provided with a white highly-reflectant surface for integrating
light scattered from a particle on the wafer surface, and for
reflecting the scattered light to the detectors 50 and 52. A set of
coordinate axes 56 shows X,Y and Z axes, the mechanical motion of
the wafer 26 being along the Y axis and the optical scanning of the
laser beam being along the X axis in a plane parallel to the Z
axis. Thereby, the scanner 22 and the conveyor 24 cooperate to
perform a two-dimensional scan of the surface of a wafer 26.
Also shown in FIG. 2 is a portion of an optical system of the
scanner 22, which optical system will be described in further
detail in FIG. 3. The portion of the optical system shown in FIG. 2
includes a telecentric scan lens 58, a rotating polygon mirror 60
(shown in phantom), a motor 62 which rotates the polygon mirror 60,
a shaft-angle encoder 64 which is mechanically coupled to the motor
62 and the mirror 60 for outputting an electric signal indicating
angular position of the mirror 60, a sealed housing 66 which
encloses the polygon mirror 60 and supports the motor 62 and the
encoder 64, and a collimating lens 68. The collimating lens 68
directs laser light towards the rotating polygon mirror 60 which,
in turn, redirects the laser light as a scanned laser beam through
the scan lens 58 and the slot 54 to the surface of the wafer 26.
The scan lens 58 is positioned above the hemisphere 48 for
directing and focusing the scanned beam through the slot 54 to the
wafer surface.
FIG. 3 shows an optical system 70 of the scanner 22 (also indicated
in phantom in FIG. 1). A portion of he components of the optical
system 70 have already been described in FIG. 2, these components
being the collimating lens 68, the polygon mirror 60, the scan lens
58, and the hemisphere 48 with its slot 54 and detectors 50 and 52.
The scan lens 58 is shown to comprise, by way of example, a front
lens element 72, and a rear doublet lens element 74. The optical
system 70 further comprises an argon ion laser 76 which produces a
10 milliwatt beam 78 of radiation at a wavelength of 4880
angstroms. The beam 78 is linearly polarized and passed through a
polarizing beam splitter 80 and a quarter-wave plate 82 which
transforms the beam into circularly polarized light. The circularly
polarized light is preferred because the circular polarization
improves detectability of small particles as compared to plane
polarized light. The beam then passes through an acousto-optic
modulator 84 which deflects approximately 90% of the beam into a
first order Bragg diffraction angle which passes into a microscope
objective 86. The remaining 10% of the light of the beam is
deflected into a beam stop 88.
The light passing through the objective 86 comes to a focus and
then expands to fill the collimating lens 68. Rays of the beam are
made parallel by the collimating lens 68 resulting in a beam being
outputted by the collimating lens 68 with a diameter which is
preferably approximately 30 millimeters in the preferred embodiment
of the invention. The beam outputted by the collimating lens 68
strikes the rotating polygon mirror 60 and is reflected by the
mirror 60 into a flat field of the telecentric scan lens 58. The
lens 58 focuses the beam to a focal spot having a diameter of
preferably 12 microns, the focal spot being at the location of a
contaminating particle 90 on the top surface of the wafer 26.
As each facet 92 of the mirror 60 rotates past the incident laser
beam, another scan line is generated upon the surface of the wafer
26. While various rotation speed of the mirror 60 are possible, in
the preferred embodiment of the invention, the mirror 60 rotates at
approximately 250 revolutions per second, generating approximately
1000 scan lines per second in the case of the four sided polygon
depicted for the mirror 60 in FIG. 3. If the mirror 60 were shaped
as an octagon instead of the square depicted in FIG. 3, then the
number of scan lines would be doubled to 2,000 scan lines per
second. At the beginning of each new sweep of the laser spot across
the surface of the wafer 26, the laser beam is deflected within the
hemisphere 48 by a mirror 94 into a photodetector 96 which may
comprise a photodiode. The mirror 94 is secured to the hemisphere
48 by an armature (not shown). The photodetector 96 produces an
electric pulse signalling the beginning of a new line scan.
The optical system 70 further comprises a lens 98, a pinhole 100,
and a light detector 102 for viewing light reflected normally from
the surface of the wafer 26 back through the scan lens 58 towards
the laser 76. Thus, there are three sets of optical sensing devices
which sense light of the laser 76 reflected from the wafer 26, as
follows. The detectors 50 and 52 in combination with the reflecting
inner concave surface of the hemisphere 48 sense scattered rays of
light reflected from a particle, such as the particle 90 on the
surface of the wafer 26. Light reflected by a flat smooth region of
the wafer surface is sensed by the detector 102. Light at an
extreme end position of the scanned laser beam is intercepted by
the mirror 94 and directed to the photodetector 96 to indicate that
another scan line is to be initiated by the rotating polygon mirror
60.
In operation, the sites of the wafer surface that are to be
inspected for particulates, are structured as bare film or silicon
that have a higher specular reflection coefficient than the rest of
the wafer surface. These sites are detected as follows. Since the
scan lens 58 is telecentric, light striking a mirror-like surface
on the wafer 26 is retro-reflected off of the wafer surface, back
through the scan lens 58, and off of the rotating mirror 60 to
produce a collimated beam proceeding along the same path as the
incident beam 78 generated by the laser 76. The retro-reflected
beam passes through the collimating lens 68, is focused through the
objective 86, is up-shifted in position by the acousto-optic
modulator, and then passes through the quarter-wave plate 82. At
this point the light is linearly polarized in a direction which is
rotated 90 degrees with respect to the initial polarization of the
retro-reflected light, this resulting in a deflection of the
retro-reflected light by the polarizing beam splitter 80 into the
lens 98. The retro-reflected light is directed by the lens 98
towards the pinhole 100 which pinhole is located at one focal
length from the lens 98. The arrangement of the lens 98 and the
pinhole 100 is confocal, and operates such that only light
reflecting off of the wafer surface is at such an image distance
that the retro-reflected light is reimaged through the pinhole 100
onto the detector 102. The pinhole 100 prevents transmission of
retroreflected light to the detector 102 from retroreflections
which may occur from surfaces other than the wafer surface such as,
by way of example, surfaces of the various lenses of the system 70,
a surface of the chuck 42, as well as other false indications of
reflected light. The detector 102 may be constructed as a PIN diode
for producing an electric signal proportional to the specular
reflectivity of the portion of the wafer surface being illuminated
by the laser beam. Thereby, the pinhole 100 prevents false
indications of light from inducing a signal from the detector
102.
The acousto-optic modulator is activated by an electric signal as
will be described subsequently with reference to FIG. 4, and
generates an acoustic wave which interacts, in the manner of an
optical grating, with the laser beam to offset the direction of
rays of light passing through the modulator 84. Such offset alters
the location of the beams incident and reflected from a facet of
the polygon mirror 60, as well as the site of the scan line of the
laser beam illuminating the wafer surface. With reference to the
coordinate axes 56 of FIG. 2, the offset of the scan line is
opposite to the direction of motion of the conveyor track 36, and
is thus an offset along the Y axis. The offset is a sideward
shifting of the position of the scan line which is parallel to the
X axis.
For example, on even scan lines, the modulator 84 is operated at an
acoustic frequency of 77.23 megahertz which operating frequency
produces a deflection in the illuminating beam of 2.744 degrees. On
odd scan lines, the modulator 84 is operated at an acoustic
frequency of 77.25 megahertz, which operating frequency produces a
beam deflection of 2.745 degrees. As a result, even scan lines on
the wafer surface are displaced 6 microns with respect to the
position of the odd scan lines.
The offsetting of the foregoing even and odd scan lines is
demonstrated in FIG. 5 wherein an even scan line 104 is shown as a
solid line and an adjacent shifted even-scan line 106 is shown as a
dashed line. The amount of shifting of the scan lines is dependent
on the selection of acoustic frequencies at the modulator 84. The
frequencies are selected in accordance with the selection of speed
of the conveyor track 36 such that the amount of offset is equal to
the amount of travel of the wafer 26 between scan lines. Thus, in
the case of the foregoing example of the preferred embodiment of
the invention wherein the offset is 6 microns per scan the movement
of the wafer 26 along the Y axis during the time interval from the
beginning of one scan to the beginning of the next scan is also 6
microns. The action of the modulator 84 is therefore to produce
pairs of overlapping scan lines, such that every other scan is 12
microns apart.
With reference also to FIG. 4, the scanner 22 further comprises an
electronics unit 108 (also indicated in phantom in FIG. 1) having
electrical circuitry and connections to components of the optical
system 70 as shown in FIG. 4. Both the system 70 and the unit 108
are indicated in phantom within the scanner 22 of FIG. 1. FIG. 4
shows components of the optical system 70 previously described with
reference to FIGS. 2 and 3, these components being the shaft angle
encoder 64, the start-of-sweep photodetector 96, the detector 102
of the bright field of the laser beam, the scattered-light
detectors 50 and 52, and the acousto-optic modulator 84. Also shown
are connections to the analyzer 40 of FIG. 1. The electronics unit
108 comprises a counter 110 connected to a count-preset encoder
112, a frequency divider 114, an AND gate 116, a CCD (charge
coupled device) analog shift register 118, an amplitude
discriminator 120, an AND gate 122, a single-pole double-throw
analog switch 124, a voltage-controlled oscillator (VCO) 126, a
pair of amplitude discriminators 128 and 130, a coincidence
detector 132 and a summer 134. In operation, the encoder 64 is
mounted on a shaft 136 (FIG. 3) of the rotating mirror 60, and
produces a pulse train at a repetition frequency dependent of the
speed of rotation of the mirror 60. At the rotation speed in the
preferred embodiment of the invention, the rate of occurrence of
the pulses of the encoder 64 is such that approximately one pulse
occurs for every 12 microns of travel of a spot of the scanned
laser beam on the surface of the wafer 26. The train of pulses
outputted by the encoder 64 serves as clock pulses for driving the
counter 110 and the register 118. The clock pulses are connected
directly to the counter 110, and are connected via the gate 116 to
the register 118. While the register 118 may be of any desired
size, in the preferred embodiment of the invention. the register
118 is provided with 16,384 cells.
The bright field detector 102 provides electric signals which are
coupled via a buffer amplifier 138 to a data input terminal of the
register 118. A data output terminal of the register 118 applies
data samples to the discriminator 120. Upon each occurrence of a
clock pulse applied via gate 116 to a clock input terminal of the
register 118, the register 118 stores the input analog value of the
signal outputted by the detector 102 in a first cell of the
register 118. Signals stored in each of the cells are shifted to
the next cell, and the signal of the last cell of the register 118
is outputted to the discriminator 120.
Data acquired by the detector 102 from specularly reflected light
is stored in the register 118 with each scan of the scanned laser
beam. Operation of the register 118 is synchronized with the
scanning of the laser beam by means of the counter 110. The counter
110 is preset at the beginning of each scan with the number stored
in the encoder 112 in response to a reset signal outputted by the
detector 96. The signal outputted by the detector 96 is applied via
a buffer amplifier 140 to a reset terminal of the counter 110. At
the start of each scan, the counter 110 is preset to a count of
16,383. In response to the sequence of clock pulses from the
encoder 64, the counter 110 counts down from the preset value until
the count of the counter 110 becomes negative. Thereupon, the most
significant bit (MSB) of the output count of the counter 110
becomes a logic-1. The MSB of the output count is applied to a
complemented input terminal of the gate 116. Therefore, the MSB
value of Logic-1 is converted to a logic-0 which deactivates the
gate 116 to stop the flow of clock pulses to the register 118. This
stops operation of the register 118. Upon resetting of the counter
110 for the next line scan, the MSB of the output count returns to
a value of logic-0 to enable the gate 116 to resume passage of
clock pulses to the register 118.
The train of reset pulses outputted by the detector 96 is applied
also to the divider 114 which divides the repetition frequency of
the pulse train by a factor of two. A train of pulses outputted by
the divider 114 is applied to an input terminal of the gate 122 and
to a control terminal of the switch 124. The gate 122 couples
signals, outputted by the discriminator 120, via a pair of buffer
amplifiers 142 and 144 to control grids 146 in the photomultiplier
tubes of the detectors 50 and 52. The grids 146 interact with a
light sensing electrode 148, a photocathode, in each of the
photomultiplier tubes to prevent the outputting of a signal by the
detectors 50 and 52 at all times except when the grids 146 are
activated with suitable gate signals applied via the amplifiers 142
and 144 from the gate 122. It is the function of the amplitude
discriminator 120 to distinguish the presence of differing
intensities of reflections of the laser beam from the wafer surface
so as to disable the detectors 50 and 52 in the presence of
undulations in the wafer surface, and to enable the detectors 50
and 52 in the presence of a flat smooth wafer surface. Accordingly,
the discriminator 120 outputs an enable gate pulse via the gate 122
to the control grids 146 in the presence of intense reflections, as
will be described below, the enable gate pulses being manifested as
a logic-1 signal outputted by the gate 122.
A feature of the invention is the pairing of scan lines such that a
first line of each pair is employed for investigating the surface
of the wafer 26 to determine which portions of the surface may
generate intense reflections in the directions of the detectors 50
and 52, and which portions of the surface are flat and smooth so as
to specularly reflect the laser beam back through the scan lens 58
without illuminating the detectors 50 and 52. In the case of the
particle 90, the illumination of the detectors 50 and 52 is a low
intensity scattered radiation. For example, it has been observed
that surface metallization of the wafer 26 can create a reflected
light beam with 10,000 times the scattered light intensity from a
10 micron spot on the wafer surface than a typical one-micron
particle. During the second scan line of each pair of lines, the
detectors 50 and 52 are activated to sense the scattered radiation
associated with a particle such as the particle 90 on the wafer
surface.
The requisite logic for accomplishing the foregoing procedures is
accomplished by the discriminator 120, the divider 114, and the
gate 122. The signal outputted by the divider 114 deactivates the
gate 122 during the first line scan of each pair of line scans so
as to prevent enablement of the detectors 50 and 52 during the
initial investigation of the wafer surface. The signal outputted by
the divider 114 activates the gate 122 to pass the enable signal
from the discriminator 120 during the second scan line in each pair
of scan lines. The register 118 stores a complete history of
retroreflected light received at the detector 102 during the first
scan line of the pair of scan lines. A strong signal stored in a
cell of the register 118 indicates that substantially all of the
laser light has been reflected back through the scan lens 58, this
showing that a smooth flat region of the wafer surface is present
at the specific location corresponding to the cell of the register
118 in which this information is stored. In the event that a cell
of the register 118 stores a signal of reduced intensity, such
reduced intensity is understood to be caused by undulations in a
patterned region of the wafer surface. Such undulations may direct
a major portion of the laser light at a direction inclined to a
normal of the wafer surface resulting in a possible illumination of
one of the detectors 50 and 52 with intense radiation. Accordingly,
a reduced signal intensity stored in a cell of the register 118 is
taken as an indication that the corresponding site on the wafer
surface is not suitable for the viewing of particles by integrated
scattered light within the hemisphere 48. The discriminator 120 is
set to output an enable signal via the gate 122 for all
retro-reflected optical signals having an intensity greater than a
preset threshold intensity. For lower intensity signals, the
discriminator 120 outputs a signal of the complementary logic state
to disable the detectors 50 and 52.
Accordingly, for each scan line, the register 118 stores a complete
record of the regions providing strong and weak reflections. In
terms of odd and even scans, the odd scan being the first scan in
each scan pair, and the even scan being the second scan in each
pair, the data accumulated by the register 118 during an odd scan
is outputted during the next even scan and evaluated by the
discriminator 120 for enablement and disablement of the detectors
50 and 52. Data accumulated during an even scan in the register 118
is outputted during the next odd scan wherein the detectors 50 and
52 are disabled by the logic operation of the gate 122. Thus, data
accumulated in the register 118 during even scans is discarded.
The switch 124 is activated by the signal outputted by the divider
114 to assume alternate ones of two possible switch states.
Thereby, the switch 124 couples a voltage from a source V.sub.1 to
a control terminal of the oscillator 126 during each odd scan to
select an oscillation frequency of the oscillator 126. During each
even scan the switch 124 couples a voltage of a voltage source
V.sub.2 to the oscillator 126 to select an alternate oscillation
frequency. The voltages of the two sources V.sub.1 and V.sub.2 have
the requisite voltages for operating the oscillator 126 at the
aforementioned frequency of 77.25 megahertz for the odd numbered
scans, and at 77.23 for the even numbered scans. The oscillator 126
outputs a sinusoidal electric signal of the desired frequency to a
control terminal of the acousto-optic modulator 84 to attain the
desired beam deflections, one deflection being attained on the odd
scans and the other deflection being attained on the even
scans.
In the construction of the photomultiplier tubes of the detectors
50 and 52, the control grids 146 are operative with a voltage pulse
of 5 volts, this being a convenient value for use with
semiconductor logic circuitry such as the gate 122. The
photomultiplier tubes can be gated on and off by logic pulses in a
sufficiently short interval of time, typically a few nanoseconds,
so as to enable a high resolution viewing of the surface of the
wafer 26 during each scan line.
When the photomultiplier tubes are activated, the combination of
the scattered light gathered by the integrating hemisphere 48
provides a measure of the total integrated scatter of laser light
from the site of the wafer surface being viewed. Output signals of
the detectors 50 and 52 are coupled via buffer amplifiers 150 and
152, respectively, to the discriminators 128 and 130. Output
signals of the discriminators 128 and 130 are coupled via the
coincidence detector 132 as a gate signal for operation of the
analyzer 40. The use of plural detectors 50, 52 rather than a
single detector, and the sensing of a coincidence in their signals
provides immunity to noise in the light detection process. The
output signals of the detectors 50 and 52 are also applied via the
amplifiers 150 and 152 to the summer 134, the summer 134 summing
together the signals of the detectors 50 and 52 to apply an analog
signal representing the entire reflected radiation of the particle
90 to the analyzer 40.
Outputting of particulate data by the analyzer 40 is accomplished
as follows. If the test site is free from particulate contaminants,
the net scatter signal is relatively low, and the signals outputted
by the detectors 50, 52 are below the amplitude threshold level of
the discriminators 128 and 130. Hence, the discriminators 128 and
130 each output a logic-0 signal to the coincidence detector 132.
If the laser light strikes a particle on the wafer surface, the
total integrated light scatter within the hemisphere 48 is
relatively high resulting in signals outputted by the detectors 50
and 52 which are above the amplitude threshold levels of the
discriminators 128 and 130. Thus, each of the discriminators 128
and 130 output a logic-1 signal to the detector 132. The scattered
light is sensed by both of the detectors 50, 52 at the same instant
of time so as to cause the detector 132 to respond to the joint
occurrence of logic-1 signals of the discriminators 128 and 130 by
outputting a gate signal which enables the analyzer 40. In response
to the gate signal of the detector 132, the analyzer 40 records the
amplitude of the particle signal inputted to the analyzer 40 by the
summer 134. The shaft angle encoder 64 outputs a train of pulses
during each scan line, the number of pulses being equal to the
number of cells of the register 118 into which the data of the
detector 102 is to be stored. Since the same pulse train is applied
to the counter 110, the count thereof is representative of the
specific locations of a scan line which is being viewed by either
the detector 102 or by the pair of detectors 50 and 52.
Accordingly, the output count of the counter 110 is also applied to
an input terminal of the analyzer 40 to indicate the location of
the site of a particle on the wafer surface for which the particle
signal of the summer 134 is being provided. Thereby, the analyzer
40 identifies the location of each particle and also shows the
characteristic of reflected light from each particle. This
information permits generation of a statistical analysis of the
particulate contaminants both in terms of the reflectivity of the
particles and the locations of the particles. It is noted that such
information is obtained independently of the specific orientation
of the wafer 26, there being no need to align a kerf region thereof
with a scan line. Also, it is noted that information is attained
from smooth flat areas which may be found occasionally within a
patterned region as well as the more numerous flat surface areas
found within a kerf region.
In the preferred embodiment of the invention, the inspection system
20 provides for a 195 millimeter field of view, which comprises a
12 micron-laser beam spot size by 16,384 pels (picture elements)
per line. The scanner 22 can scan an 8 inch diameter wafer in 34
seconds, this being a distance of 200 millimeters which is viewed
at a rate of 6 millimeters per second. The system 20 can detect
unpatterned high reflectivity regions as small as 36 microns, this
having an area equal to the size of 3 spots, and is capable of
detecting particles in these regions equal to or larger than
approximately 0.3 microns in diameter. The particle counts are
accumulated and sorted by size, based on scattered light intensity,
by the analyzer 40.
By way of summary of the operation of the inspection system 20,
FIG. 6 shows the method steps in the operating procedure of the
system 20. As set forth in FIG. 6, the wafer is made to advance in
a first direction after which the surface of the wafer is optically
scanned in a second direction, transverse to the first direction.
the intensities of light retroreflected through the optical system
70 from the wafer surface during the scan are recorded in the shift
register 118 as a function of light beam location on the wafer
surface. Locations are given by the sequence of clock pulses
emanating from the shaft angle encoder 64 connected to the rotating
polygon mirror 60. There follows an introduction of an offset in
the optical system to compensate for wafer motion, which offset is
accomplished by the acousto-optic modulator 84. The wafer surface
is then rescanned following the same path as the previous scan. The
detectors 50 and 52 in the hemisphere 48 are then activated via the
AND gate 122 at the locations in which high intensity
retroreflections were obtained during the previous scan. There
follows a recording of the intensities of scattered light from
particulate contaminants as detected by the hemisphere detectors.
The recording of data from the hemisphere detectors is accomplished
as a function of light beam location on the wafer surface, the
location being given by the count of the counter 110. Thereupon,
the offset introduced by the acousto-optic modulator 84 is removed
and the foregoing two scans are repeated to gather data about
another region of the wafer. Upon completion of the scanning of the
complete wafer, the particle data is evaluated by the analyzer
40.
It is to be understood that the above described embodiment of the
invention is illustrative only, and that modifications thereof may
occur to those skilled in the art. Accordingly, this invention is
not to be regarded as limited to the embodiment disclosed herein,
but is to be limited only as defined by the appended claims.
* * * * *